1. Field of the Invention
The present invention relates generally to airborne radar antennas and more particularly to an electronically scannable antenna especially adapted for mounting in the wing of small aircraft.
2. Description of the Prior Art
Weather avoidance radar systems are widely used in multi-engine aircraft and have, in the past, utilized some form of reflector antenna such as a parabolic or a flat plate dish antenna which may be scanned mechanically. Due to the size of such antennas and the requirement for mechanical scanning, it is common to mount such antennas in a radome attached to the nose of multi-engine aircraft. However, in most single engine aircraft, there is no nose section available for mounting a weather avoidance scanning radar antenna since this is the location of the aircraft engine. Due to the large number of weather-related aircraft accidents involving small private planes, a number of attempts have been made to install weather avoidance radar scanning antennas in the leading edge of the wing of single engine aircraft. For example, RCA Avionics Systems has developed an X-band weather radar, known as the WeatherScout I, which uses a truncated parabolic scanning antenna which may be installed in the leading edge of an aircraft wing. However, this solution has been marginally acceptable. The moving parts of the mechanically scanned antenna are subject to freezing problems, frequent maintenance and repair, and normal wear of the mechanisms. The antenna performance is not completely satisfactory. The azimuth side lobes are only about 18 dB down from the main lobe and 13 dB down in elevation. The beamwidth is about 8.degree. which limits the resolution.
The requirement to mount the scanning antenna in the leading edge of the wing suggests that a phased array which can be electronically scanned presents the greatest promise. However, the size of known phased arrays which will produce the necessary narrow beamwidth antenna have been found to be excessive. One type of electronically scanned array has appeared which has a suitable configuration and is described in U.S. Pat. Nos. 3,008,141 to Cohn, et al and 3,039,097 to Strumwasser, et al. The antenna arrays in this prior art are formed from folded waveguide sections having multiple radiating ports arranged such that the relative phase of the radiated energy from the ports will determine the angle of the radiated beam and the number of ports and distribution of amplitudes of energy from the ports will determine the beamwidth. In general, these waveguide antennas have certain fixed selected lengths of waveguide associated with each radiating port such that a zero azimuth beam is produced when all radiated waves are in phase. The frequency of the input electromagnetic energy is varied linearly over a selected range resulting in phase shifts with respect to each radiating port. Thus, a proper selection of scan frequencies will permit scanning of the beam in azimuth.
In the Strumwasser patent, radiation takes place from slots cut in the top surfaces of the multiplicity of parallel waveguide sections connected at each end with 180.degree. E-bends producing a narrow beam in azimuth. The radiation from the series of waveguide slots is reflected from a curved plate which serves to limit the vertical beamwidth. The spacings between the parallel waveguide sections disclosed in these patents are such that a very long array is required to obtain a sufficiently narrow azimuth beam for the desired resolution. Furthermore, the distance between the radiating slots is, by necessity on the order of one wavelength which produces a satisfactory pattern when the beam has a zero azimuth angle. However, for scan angles much greater than 25.degree. or so, this wide spacing produces severe side lobes.
In the Cohn antenna, a similar folded waveguide structure is disclosed with apertures in the waveguide that couple into an array of closed-end waveguide sections having small flared horns at the outer, open ends. Thus, radiation takes place from the parallel array of flared horns. As in the Strumwasser antenna, the radiating sections are widely spaced. This is necessary to accommodate the flared portion of the radiating horns, and center-to-center spacings of about one wavelength are found. Although the curved plate reflector of Strumwasser is eliminated, the length of the array for a very narrow radiated beam is also excessive for aircraft application and the side lobe problem exists for large scan angles.
The excessive size and poor side lobe characteristics of these known scanning electronically scanned arrays make them unsuitable for wing mounting in small aircraft.